Cottonwoods & Caterpillars

The caterpillar of the western tiger swallowtail butterfly is a known pest of cottonwood.

It’s hard to accept that one of the most treasured and least feared group of insects could be responsible for the destruction to such valuable assets as trees. That colorful butterfly flitting around your garden or that harmless moth circling your patio lantern was likely, in its immature phase, damaging a nearby tree. Caterpillars, the larval form of butterflies and moths, typically feed on plants. Most trees, including cottonwood and other Populus species, are susceptible to these pests.

Cottonwoods

Named for the cottony seeds that the female tree produces, cottonwood trees belong to the genus Populus in the Salicaceae, or willow, family. This deciduous tree is present throughout North America with one particular species native to U.S. Department of Agriculture plant hardiness zones 8 through 10. Freemont’s cottonwood (Populus fremontii) reaches heights over 100 feet and typically grows near rivers, streams and wetlands. Bark on young trees is smooth, but it's deeply furrowed on aging trees. The leaves are heart shaped with serrated edges. Cottonwoods help to control erosion on stream banks and improve water quality.

Wood-Boring Caterpillars

While some caterpillars feed exclusively on soft plant tissue like leaves, flowers and fruit, a few cottonwood pests bore into the wood. The western poplar clearwing and the American hornet moth belong to the clearwing moth family Sesiidae. As adults, these moths are often mistaken for wasps due to their wing shape, body coloring and flight behavior. Caterpillars of these clearwings are 1 1/2 inches long with a creamy white body and black head. The carpenter worm is another wood-boring pest of cottonwood. This moth is mottled gray with a wingspan of up to 3 inches. Its larvae are thick, greenish-white caterpillars with a dark head. They can grow up to 3 inches long.

Foliage-Feeding Caterpillars

Hundreds of moth and butterfly caterpillars are generalist feeders eating the leaves of a wide variety of species. Some of these generalists are known pests to cottonwood. The western tussock moth caterpillar is distinctive with a pair of long hair tufts protruding from the front of their bodies and four white tufts on its back. The western tent caterpillar and the fall webworm are easily identified by their congregating behavior in large silken tents on infested trees. Soft-bodied, hairless caterpillars that feed on cottonwood include the small, white fruit tree leafroller and the large, green western tiger swallowtail. Further cottonwood pests include the red humped caterpillar, the satin moth caterpillar and the spiny elm caterpillar.

Damage and Treatment

The damage to cottonwoods from foliage-feeding caterpillars is usually obvious as leaves are rolled or chewed. Where possible, hand remove the pests and their eggs and dispose of them in soapy water. Predators such as parasitic wasps are commercially available and are effective against caterpillars. A spray application of the bacterium Bacillus thuringiensis on young caterpillars is also helpful in reducing or eliminating leaf-eating larvae. Dying limbs, sawdust around holes in the trunk and insect droppings protruding from holes are all indicators of a wood-boring pest. Once the larvae are inside the wood, chemical treatments for the pest are ineffective. The best control against wood-boring caterpillars is prevention. Sticky pheromone traps hung in the tree help prevent males from mating with females. Inspect the bark frequently for insect eggs and remove them by scraping or brushing into a bucket of soapy water.

A Field Guide to the Insects: America North of Mexico; D.J. Borror and R.E. White

About the Author

Jean Godawa is a science educator and writer. She has been writing science-related articles for print and online publications for more than 10 years. Godawa holds a degree in biology and environmental science with a focus on entomology from the University of Toronto. She has conducted field research in the tropical rainforests of southeastern Asia and South America.